Optical transmitter, optical transmission device, and transmission method

Abstract

An optical transmitter includes a signal-processing circuit configured to perform signal processing on a first transmission signal and output a second transmission signal; an optical modulator configured to modulate input light with the second transmission signal and to output an optical signal; and a control circuit configured to output a control signal for controlling a carrier frequency of the optical signal to the signal-processing circuit, wherein the signal-processing circuit includes a map-adjustment circuit configured to adjust, based on the control signal and a modulation format, a map position of the second transmission signal onto a complex plane, and a phase-rotation circuit configured to apply, on the complex plane, rotation of a phase of the carrier frequency corresponding to the control signal to the second transmission signal at the adjusted map position.

What is claimed is: 
     
         1 . An optical transmitter comprising:
 a signal-processing circuit configured to perform signal processing on a first transmission signal and output a second transmission signal;   an optical modulator configured to modulate input light with the second transmission signal and to output an optical signal; and   a control circuit configured to output a control signal for controlling a carrier frequency of the optical signal to the signal-processing circuit,   wherein the signal-processing circuit includes
 a map-adjustment circuit configured to adjust, based on the control signal and a modulation format, a map position of the second transmission signal onto a complex plane, and 
 a phase-rotation circuit configured to apply, on the complex plane, rotation of a phase of the carrier frequency corresponding to the control signal to the second transmission signal at the adjusted map position. 
   
     
     
         2 . The optical transmitter according to  claim 1 ,
 wherein, when the control signal indicates displacement of the carrier frequency, the map-adjustment circuit adjusts the map position of the second transmission signal, based on an arrangement pattern in which symbol points on an outermost shell that are determined according to the modulation format are in a range of an inscribed circle of a dynamic range upper limit of the optical transmitter and a distance between the symbols is largest.   
     
     
         3 . The optical transmitter according to  claim 2 ,
 wherein the signal-processing circuit has a memory that stores the arrangement pattern therein; and   wherein, when the control signal indicates displacement of the carrier frequency, the map-adjustment circuit adjusts the map position of the second transmission signal by using the arrangement pattern stored in the memory.   
     
     
         4 . The optical transmitter according to  claim 2 ,
 wherein the arrangement pattern is a pattern in which all inter-symbol distances are set to an equal distance through close-packed arrangement of hexagons on the complex plane.   
     
     
         5 . The optical transmitter according to  claim 4 ,
 wherein the hexagons have symbol points at respective centers thereof; and the arrangement pattern is a pattern in which, in the close-packed arrangement of the hexagons, a smallest circle having a center point at one of the symbol points and including at least a number of symbol points which is determined by the modulation format is set to a symbol arrangement area, and a position of the center point is adjusted so that a number of symbol points which corresponds to a difference between the number of symbol points included in the smallest circle and a number of symbol points which is determined by the modulation format lie outside the smallest circle.   
     
     
         6 . The optical transmitter according to  claim 2 ,
 wherein, in the arrangement pattern, symbol points are arranged on a plurality of circles having a same center on the complex plane, and each symbol point is arranged at an equal distance from a closest symbol point that lies on the inner or outer circle and that is adjacent to the symbol point.   
     
     
         7 . The optical transmitter according to  claim 6 ,
 wherein the arrangement pattern is determined with reference to a distance between the adjacent symbols when the innermost circle is equally divided by four symbol points.   
     
     
         8 . The optical transmitter according to  claim 6 ,
 wherein the arrangement pattern is a pattern in which the outermost circle corresponds to the inscribed circle of the dynamic range upper limit.   
     
     
         9 . An optical transmission device comprising:
 optical transmitters, each including
 a signal-processing circuit configured to perform signal processing on a first transmission signal and output a second transmission signal, 
 an optical modulator configured to modulate input light with the second transmission signal and to output an optical signal, and 
 a control circuit configured to output a control signal for controlling a carrier frequency of the optical signal to the signal-processing circuit; and 
   a multiplexer configured to multiplex optical signals output from the optical transmitters, the optical transmitters being configured to output the optical signals having carrier frequencies that are different from each other,   wherein the signal-processing circuit includes
 a map-adjustment circuit configured to adjust a map position of the second transmission signal onto a complex plane, based on the control signal and a modulation format, and 
 a phase-rotation circuit configured to apply, on the complex plane, rotation of a phase of the carrier frequency corresponding to the control signal to the second transmission signal at the adjusted map position. 
   
     
     
         10 . A transmission method comprising:
 adjusting, by a signal-processing circuit, a map position of a transmission signal onto a complex plane when displacement of a carrier frequency of an output signal output from a transmitter is detected; and   applying, on the complex plane, rotation of a phase of the carrier frequency corresponding to an amount of the displacement of the carrier frequency to the transmission signal at the adjusted map position.